1. Field of the Invention
This invention relates in general to an improved technology for a pyramid-shaped yoke portion that is formed to be continuous to a body portion of a funnel for a cathode ray tube (CRT).
2. Description of Related Art
As is well known, a cathode ray tube mainly comprises a panel having a substantially rectangular image display portion and a funnel that is sealed to the panel to allow electron beams to pass therein. The funnel comprises a large opening portion with a substantially rectangular shape, a small opening portion with a substantially circular shape to which a cylindrical neck tube for installing an electron gun therein is connected, a substantially funnel-shaped body portion, and a yoke portion, on which a deflection yoke is externally mounted, being continuous to the body portion at the side of the small opening portion.
In the cathode ray tube with the above elements, because the deflection yoke is a large consuming source of electric power, it is very important to reduce the consumption of electricity of the deflection yoke to save and reduce the total consuming power of the cathode ray tube. To reduce the consuming power of the deflection yoke, an approach is known where the neck tube of the funnel can be shrunk to a small diameter and the outer diameter of the yoke portion on which the deflection yoke is mounted is reduced.
However, because the electron beams, which are emitted from the electron gun inside the neck tube, pass nearby the inner wall of the yoke portion, as the outer diameter of the neck tube or the yoke portion is reduced, the electron beams, which directs toward the diagonal portion that forms a maximum deflection angle, might collide with the inner wall of the yoke portion. As a result, there is a problem that a good image cannot be obtained.
In order to solve such a problem, a pyramid shape, which changes gradually from circular shape to a substantially rectangular shape in the direction from the neck tube side to the panel side, has been practically used. If the outside of the yoke portion is the pyramid shape, the diameters in the major and minor axis directions of the deflection yoke can be reduced, and therefore, the deflection coils corresponding to the major and minor axis directions can approach the electron beams. In this way, the electron beams can be more efficiently deflected, and advantageously, the deflection power can be reduced.
Similarly, in order to keep the electron beams, which directs toward the diagonal portion of the inner diameter of the yoke portion, from colliding with the internal wall thereof, a pyramid shape, which changes gradually from circular shape to a substantially rectangular shape in the direction from the neck tube side to the panel side, has been practically used.
In this situation, for example, referring to Japanese Laid Open No. 11-120940 or No. 11-176355, when the inner face of the yoke portion is formed with a pyramid shape, the diagonal portion is formed with an arc-shaped curved portion in consideration of the moldability or the strength. The curved portions at the major and minor axis sides are circular arc shapes with substantially the same radii with respect to the diagonal axis as a reference. It is usual that the shape of the inner face of the yoke portion is substantially corresponding to an ideal passing area of the electron beams.
The diagonal axes of the yoke portion are generally set corresponding to a passing area of the electron beams.
However, distortions often occur in the actual passing area of the electron beams due to the accuracy of the deflection yoke and the like. Therefore, the diagonal axis of the yoke portion, in fact, has a directional deviation with respect to the passing area of the electron beams.
In the cathode ray tube having the above characteristic, if the curved portion formed at the diagonal portion of the yoke portion has arc shapes with substantially the same curvature radii at the major axis side and the minor axis side with respect to the diagonal axis as a reference, the electron beams with a maximum deflection angle will collide with a partially curved portion at any one side thereof. Therefore, a good image cannot be obtained.
In order to solve this problem, as shown in FIG. 8, a diagonal inner-face curved portion 8 and a diagonal outer-face curved portion 9 of a diagonal portion 7 are expanded by a single circular arc, while a constant distance between the diagonal portion 7 of the yoke portion 6 and the tube axis is maintained. However, the design and manufacture of a mold for the funnel, the design and manufacture for the deflection yoke, and a variety of other portions related to producing the cathode ray tube have to be completely changed. Thus, a lot of trouble or effort is caused and the cost becomes large.
Alternatively, to thin the thickness of the yoke portion entirely has also been considered. But, if the thickness is made too thin, there are problems in molding or strength, so that there is a limitation to thinning the thickness of the yoke portion and this strategy cannot be an effective solution.
According to the foregoing description, it is an object of the present invention to provide a funnel for a cathode ray tube, wherein by simply and effectively improving a diagonal portion of a yoke portion, it is not necessary to basically modify the mold or the deflection yoke, etc. and therefore, the electron beams can be suitably kept from colliding with the yoke portion, so as to be able to obtain a good image.
In order to achieve the object, the present invention provides a funnel for a cathode ray tube having a large opening portion with a substantially rectangular shape, a small opening portion with a substantially circular shape to which a cylindrical neck tube is connected, a funnel-shaped body portion continuous from the large opening portion, and a pyramid-shaped yoke portion continuous from the body portion to the small opening portion, comprising a diagonal inner-face curved portion is formed on an inner face of a diagonal portion of the yoke portion, wherein the diagonal inner-face curved portion comprises a partially inner-face curved portion at a major axis side and a partially inner-face curved portion at a minor axis side, both of which are imaginarily divided to each other with respect to a diagonal axis as a reference, and wherein at least one of the partially inner-face curved portions at the major axis side and the minor axis side swells to the outward side.
The term xe2x80x9cswells to the outward sidexe2x80x9d means that at a cross-sectional plane perpendicular to the tube axis, at least one of the partially inner-face curved portions swells to the outward side (to the outer-face side of the yoke portion) than an imaginary extension line that extends along the inner face of the major side or the minor side, which form the yoke portion, toward the diagonal axis side. Therefore, when the inner faces of the minor side and the major side are respectively formed by circular arcs, the term xe2x80x9cswells to the outward sidexe2x80x9d means that at least one of the partially inner-face curved portions swells to the outward side than an imaginary extension line of the circular arc. Moreover, in detail, the term xe2x80x9cswells to the outward sidexe2x80x9d means that with respect to an imaginary reference curved line (a conventional curved line) whose curvature varies slowly from a central point of the inner face of at least one of the major axis side and the minor axis side toward the diagonal axis, the partially inner-face curved portion of at least one of the major axis side and the minor axis side swells to the outward side due to the changing of the curvature thereof greater than that of the imaginary reference curved line in the halfway of the imaginary reference curved line. In addition, when one of the partially inner-face curved portions of the major axis side and the minor axis side swells to the outward side, the one partially inner-face curved portion swells to the outward side relative to the other partially inner-face curved portion.
According to the above structure, for the diagonal inner-face curved portion formed on the inner face of the diagonal portion of the yoke portion, because both of or any one of the partially inner-face curved portions of the major axis side and the minor axis side swell (or swells) to the outward side, even though the electron beams with a maximum deflection angle may collide with one of the partially inner-face curved portions due to the diagonal axis of the yoke portion creating a directional deviation with respect to the actual passing area of the electron beams, the partially inner-face curved portion that swells to the outward side can serve as a passage through which the electron beams can pass. Therefore, the electron beams can be avoided from colliding with the curved portion formed on the diagonal portion of the yoke portion, so that a good image can be obtained. Furthermore, because the curved portion formed on the inner face of the diagonal portion of the yoke portion is not entirely expanded, but only a portion is swelled to the outward side with making a cross position between the diagonal axis and the curved portion almost unchangeable, design and manufacture of the mold for the funnel or the deflection yoke etc. are not greatly affected. Thus, collision of the electron beams can be avoided with only a minimum modification, and therefore, trouble, effort, and cost burden required for the modification can be further reduced.
It is preferred that the partially inner-face curved portions at the major axis side and the partially inner-face curved portion at the minor axis side are formed by circular arced portions with different radius to each other. In this situation, the circular arced portion with a smaller radius may be swelled to the outward side than the circular arced portion with a larger radius, or the circular arced portion with a larger radius may be swelled to the outward side than the circular arced portion with a smaller radius. In addition, the circular arced portion at the major axis side and the circular arced portion at the minor axis side may be jointed on the diagonal axis, or on a position other than the diagonal axis. Furthermore, each of the circular arced portions at the major axis side and the minor axis side may be drawn by single circular arc respectively, or may be drawn by jointing a plurality of circular arcs respectively. According to this structure, because one of the partially inner-face curved portions swells relatively to the outward due to the difference between the radii of the circular arced portions at the major axis side and the minor axis side, the design and manufacture of the curved portion can be easily performed and the rising of the manufacturing cost can be suppressed. The xe2x80x9cjointingxe2x80x9d between the circular arced portion at the major axis side and the circular arced portion at the minor axis side means to inscribe or to substantially inscribe. For substantially inscribing the circular arced portions, it is necessary to smoothly connect the bending point between the circular arcs (following description is the same).
Preferably, the central point of one circular arced portion, the central point of the other circular arced portion, and the jointing point of the circular arced portions are set on or in the vicinity of a single straight line. Namely, in a case that the radii of the one circular arced portion and the other circular arced portion are different to each other, when the central points of the circular arced portions and the jointing point therebetween are set on the straight line, although the curvature radii of the circular arced portions are different to each other, the tangent line at the jointing point between the circular arced portions is perpendicular to the straight line, so that the jointing portion between the circular arced portions is not bending, but smoothly continued. Therefore, when the central points of the circular arced portions and the jointing point between the circular arced portions are set on or in the vicinity of the straight line, the jointing portion between the circular arced portions can be avoided from being too bending and the moldability for the jointing portion can be well maintained.
The above-mentioned invention relates to the inner face of the diagonal portion of the yoke portion. However, it is preferable that the same structure may be applied to an outer face of the diagonal portion of the yoke portion. Namely, a diagonal outer-face curved portion is formed on the outer face of the diagonal portion of the yoke portion, wherein the diagonal outer-face curved portion comprises a partially outer-face curved portion at the major axis side and a partially outer-face curved portion at the minor axis side, both of which are imaginarily divided to each other with respect to the diagonal axis as a reference. At least one of the partially outer-face curved portions at the major axis side and the minor axis side swells to the outward side. Also, in this situation, the term xe2x80x9cswells to the outward sidexe2x80x9d means that at a cross-sectional plane perpendicular to the tube axis, at least one of the partially outer-face curved portions swells to the outward side than an imaginary extension line that extends along the outer face of the major side or the minor side, which form the yoke portion, toward the diagonal axis side. Therefore, when the outer faces of the minor side and the major side are respectively formed by circular arcs, the term xe2x80x9cswells to the outward sidexe2x80x9d means that at least one of the partially outer-face curved portions swells to the outward side than an imaginary extension line of the circular arc. Moreover, in detail, the term xe2x80x9cswells to the outward sidexe2x80x9d means that with respect to an imaginary reference curved line (a conventional curved line) whose curvature varies slowly from a central point of the outer face of at least one of the major axis side and the minor axis side toward the diagonal axis, the partially outer-face curved portion of at least one of the major axis side and the minor axis side swells to the outward side due to the changing of the curvature thereof greater than that of the imaginary reference curved line in the halfway of the imaginary reference curved line. According to such structure, because the inner face of the diagonal portion of the yoke portion is partially swelled to the outward side and at the same time the outer face of the diagonal portion is also partially swelled to the outward side, the thickness of the entire diagonal portion is sufficient. As a result, although the partially swelling portion is formed, the entire rigidity of the diagonal portion can be set within a proper range.
In this situation, only one of the partially inner-face curved portions and only one of the partially outer-face curved portions at the major axis side and the minor axis side may be swelled to the outward side. According to the structure, the thickness of the portion that swells and the portion that does not swell at the diagonal portion is uniformized, so that the strength is not partially reduced and a good moldability can be maintained.